For the hottest month of the year at the proposed airport site, the monthly mean of the average daily temperature is $39^{\circ}C$. The monthly mean of the maximum daily temperature is $48^{\circ}C$ for the same month of the year. From the given information, the calculated Airport Reference Temperature $\text{(in}\: ^{\circ}C),$ is

1. $36$
2. $39$
3. $42$
4. $48$